In general, an image sensor is a semiconductor device that transforms an optical image to electrical signals. The image sensor is generally classified as a charge coupled device (CCD) or a CMOS image sensor.
The CCD type image sensor includes several MOS (metal oxide semiconductor) capacitors, closely positioned to one another, in which electric charge carriers are transferred to or saved in the MOS capacitors.
The CMOS image sensor has incorporated a switching mode by forming MOS transistors for each unit pixel with CMOS technology and using control circuits and signal-processing circuits in conjunction with the MOS transistors to sequentially detect outputs of the photodiodes.
The CCD has various disadvantages, such as a complicated driving mode, high power consumption, and inability to incorporate a signal processing circuit on-chip for the CCD due to the many mask processes.
The CMOS image sensor obtains an image from the formation of a photodiode and a MOS transistor within a unit pixel to detect signals using a switching mode. As mentioned above, because the CMOS image sensor makes use of CMOS manufacturing technology, the CMOS image sensor has low power consumption as well as a single manufacturing process requiring about 20 masks compared with the CCD manufacturing process requiring 30 to 40 masks.
As a result, the CMOS image sensor can integrate a signal processing circuit into a single chip. Accordingly, the CMOS image sensor is currently used in various applications, such as digital still cameras (DSC), PC cameras, and mobile cameras.
The CMOS image sensor is classified as a 3T type, a 4T type or a 5T type according to the number of transistors formed in a unit pixel. The 3T type CMOS image sensor includes a single photodiode and three transistors, and the 4T type CMOS image sensor includes a single photodiode and four transistors. The 3T type CMOS image sensor will now be described with reference to an equivalent circuit diagram and a lay out thereof.
FIG. 1 is an equivalent circuit diagram of a 3T type CMOS image sensor according to the related art. FIG. 2 is a layout view showing a unit pixel of a 3T type CMOS image sensor according to the related art.
As shown in FIG. 1, the unit pixel of the typical 3T type CMOS image sensor according to the related art includes one photodiode (PD) and three NMOS transistors T1, T2 and T3. The photodiode includes a cathode connected to the drain of the first NMOS transistor T1 and the gate of the second NMOS transistor T2.
Further, the sources of the first and second NMOS transistors T1 and T2 are connected to a power line that supplies a reference voltage, and the gate of the first NMOS transistor T1 is connected to a reset line that supplies a reset signal.
Also, the source of the third NMOS transistor T3 is connected to the drain of the second NMOS transistor, and the drain of the third NMOS transistor T3 is connected to a reading circuit (not shown) through a signal line. The gate of the third NMOS transistor T3 is connected to a column selection line that supplies a selection signal SLCT.
Accordingly, the first NMOS transistor T1 is a reset transistor Rx, and the second NMOS transistor T2 is a drive transistor DX. The third NMOS transistor T3 is a selection transistor Sx.
As shown in FIG. 2, an active region 10 is defined in the unit pixel of the 3T type CMOS image sensor. One photodiode 20 is formed at a wider part of an active region 10, and gate electrodes 30, 40, and 50 of three transistors are formed overlapping a remaining part of the active region 10.
Namely, a reset transistor Rx is formed by a first gate electrode 30, a drive transistor Dx is formed by a second gate electrode 40, and a select transistor Sx is formed by a third gate electrode 50.
Impurity ions are implanted in the remaining part of the active region to form source/drain regions for each transistor.
Accordingly, a power source voltage Vdd is applied to source/drain regions between the reset transistor Rx and the drive transistor Dx. Source/drain regions at one side of the select transistor Sx are coupled to a reading circuit.
The gate electrodes 30, 40, and 50 are connected to respective signal lines, and the respective signal lines are connected to an external drive circuit through a pad at one end thereof.
FIG. 3 is a cross-sectional view of the CMOS image sensor according to the related art taken along the line III-III′ of FIG. 2.
As shown in FIG. 3, a P-expitaxial layer 102 is formed on a surface of a p+-type semiconductor substrate 101 having a device isolation region and an active region defined thereon. A device isolation layer 103 is formed at the device isolation region of the semiconductor device 101 between input regions of red light, green light, and blue light.
Moreover, a gate insulating layer 104 is interposed on the transistor region of the semiconductor substrate 101 to form gate electrodes 105. Insulating sidewalls 106 are formed at both sides of the gate electrode 105, and an n−-type diffusion region 107 is formed at a photodiode region at one side of the gate electrode 105.
Furthermore, an n+-type diffusion region 108 is formed in a transistor region of the substrate at another side of the gate electrode 105, and an interlayer dielectric 109 is formed on the semiconductor substrate 101 including the gate electrode 105. A conductive plug 110 is formed to pass through the interlayer dielectric 109, and is electrically connected to the n−-type diffusion region 107 which is the photodiode region.
A metal wiring 111 is formed on the interlayer dielectric 109 in contact with the conductive plug.
The photodiode region receives light, and is a significantly important region that can be adversely affected by plasma damage during the fabrication process.
In particular, upon forming the gate electrode 105, a first plasma damage may be substantially applied to the photodiode region. Further, during the formation of the insulating layer sidewalls 106, a second plasma damage may again be applied thereto.
In addition, when the interlayer dielectric is selectively removed to form a contact hole, a third plasma damage may be applied to the photodiode region.
The multiple plasma damage applied to the photodiode region functions to generate electrons in a state free of light, thereby weakening the CMOS image sensor.